The present invention relates to the use of shock- and/or heat-sensitive organic peroxides as initiators for the polymerization of olefinic compounds.
Many organic peroxides are known and used to initiate polymerization of monomers containing olefinic unsaturation. Many such peroxides are shock sensitive and usually are handled only in diluted form. Generally, the addition of an appropriate diluent reduces the shock sensitive of the organic peroxide to the point where it can be safely handled. The problem of shock sensitivity of such peroxides has been recognize for many years. See, e.g. U.S. Pat. No. 4,178,263, which discloses diluting the peroxide with an olefinically unsaturated monomer which does not homopolymerize. According to said patent, monomers which exhibit no tendency to homopolymerize are those monomers which in admixture with an organic polymerization-inducing peroxide do not polymerize to more than one percent conversion of monomer to polymer at 100 hours at 40.degree. C. when the organic peroxide and monomer are present in a one-to-one ratio by weight. Typical non-homopolymerizing monomers include: maleic anhydride, dimethyl maleate, diethyl maleate, citriconic anhydride, dimethyl citriconate, methyl citriconate, ethyl citriconate, fumaronitrile, methyl fumuarate, dimethyl fumuarate, ethyl fumurate, diethyl fumuarate, cinnamonitrile, methyl cinnamate, ethyl cinnamate, stilbene, and the like. These monomers readily copolymerize with the basic monomer present, e.g. styrene.
When diluted peroxide compositions are employed in the free-radical polymerization of olefinically unsaturated materials, the diluent material frequently has a deleterious effect on the resulting polymer; for example, in the polymerization of polystyrenes the presence of oils, hydrocarbon diluents, high molecular weight hydrocarbons and the like are not readily removed from the polymer after polymerization and result in a reduction in the heat-distortion temperature. When polymers made with organic peroxides are used in food contact applications, the diluent used with the peroxide must be nontoxic. In many countries, approval by a government agency is required for use of the diluent in the polymer. In the case of styrenic polymers, the list of additives which meet the requirements for a good flegmatizer and which are also approved for food contact applications is very small. Examples of known flegmatizers include dioctyl phthalate, fatty acid esters, epoxidized soybean oil and mineral oil. Of these, only mineral oil and epoxidized soybean oil are approved over a wide geographic area for use in food contact applications. While mineral oil is approved for use in the preparation of polystyrene for food contact applications, it is not a good flegmatizer for some new, highly polar initiators, as they are not soluble in the relatively nonpolar mineral oil.
In view of the deficiencies of the prior art solutions to the problem of flegmatizing organic peroxides for use in the polymerization of olefinically unsaturated monomers, it would be desirable to have a flegmatizer which is suitable for food contact applications, and which has polarity sufficient to be compatible with highly polar initiators. Additionally, it would be desirable to have a flegmatizer which is an inert material, and which can easily be mixed with organic peroxide initiators to make them safe and easily handled.